Methods of fabricating gallium nitride semiconductor layers by lateral overgrowth through offset masks

ABSTRACT

Abstract of Disclosure 
     A gallium nitride semiconductor layer is fabricated by masking an underlying gallium nitride layer with a first mask that includes a first array of openings therein and growing the underlying gallium nitride layer through the first array of openings and onto the first mask, to thereby form a first overgrown gallium nitride semiconductor layer.  The first overgrown layer is then masked with the second mask that includes a second array of openings therein.  The second array of openings is laterally offset from the first array of openings.  The first overgrown gallium nitride layer is then grown through the second array of openings and onto the second mask, to thereby form a second overgrown gallium nitride semiconductor layer.  Microelectronic devices may then be formed in the second overgrown gallium nitride semiconductor layer.

Cross Reference to Related Applications

[0001] This application is a divisional of Application Serial No. 09/031,843, filed February 27, 1998, entitled Gallium Nitride Semiconductor Structures Including Laterally Offset Patterned Layers, assigned to the assignee of the present invention, the disclosure of which is hereby incorporated herein by reference.

Federal Research Statement

[0002] This invention was made with Government support under Office of Naval Research Contract No. N00014-96-1-0765. The Government has certain rights to this invention.

Background of Invention

[0003] Gallium nitride is being widely investigated for microelectronic devices including but not limited to transistors, field emitters and optoelectronic devices. It will be understood that, as used herein, gallium nitride also includes alloys of gallium nitride such as aluminum gallium nitride, indium gallium nitride and aluminum indium gallium nitride.

[0004] A major problem in fabricating gallium nitride-based microelectronic devices is the fabrication of gallium nitride semiconductor layers having low defect densities. It is known that one contributor to defect density is the substrate on which the gallium nitride layer is grown. Accordingly, although gallium nitride layers have been grown on sapphire substrates, it is known to reduce defect density by growing gallium nitride layers on aluminum nitride buffer layers which are themselves formed on silicon carbide substrates. Notwithstanding these advances, continued reduction in defect density is desirable.

[0005] It is also known to fabricate gallium nitride structures through openings in a mask. For example, in fabricating field emitter arrays, it is known to selectively grow gallium nitride on stripe or circular patterned substrates. See, for example, the publications by coinventor Nam et al. entitled "Selective Growth of GaN and Al_(0.2)Ga_(0.8)N on GaN/AlN/6H-SiC(0001) Multilayer Substrates Via Organometallic Vapor Phase Epitaxy", Proceedings of the Materials Research Society, December 1996, and "Growth of GaN and Al_(0.2)Ga_(0.8)N on Patterened Substrates via Organometallic Vapor Phase Epitaxy", Japanese Journal of Applied Physics., Vol. 36, Part 2, No. 5A, May 1997, pp. L-532-L535. As disclosed in these publications, undesired ridge growth or lateral overgrowth may occur under certain conditions.

Summary of Invention

[0006] It is therefore an object of the present invention to provide improved methods of fabricating gallium nitride semiconductor layers, and improved gallium nitride layers so fabricated.

[0007] It is another object of the invention to provide methods of fabricating gallium nitride semiconductor layers that can have low defect densities, and gallium nitride semiconductor layers so fabricated.

[0008] These and other objects are provided, according to the present invention, by fabricating a gallium nitride semiconductor layer by laterally growing an underlying gallium nitride layer to thereby form a first laterally grown gallium nitride semiconductor layer, and laterally growing the first laterally grown gallium nitride layer to thereby form a second laterally grown gallium nitride semiconductor layer. Microelectronic devices may then be formed in the second laterally grown gallium nitride semiconductor layer.

[0009] More specifically, in a preferred embodiment, a gallium nitride semiconductor layer is fabricated by masking an underlying gallium nitride layer with a first mask that includes a first array of openings therein and growing the underlying gallium nitride layer through the first array of openings and onto the first mask, to thereby form a first overgrown gallium nitride semiconductor layer. The first overgrown layer is then masked with the second mask that includes a second array of openings therein. The second array of openings is laterally offset from the first array of openings. The first overgrown gallium nitride layer is then grown through the second array of openings and onto the second mask, to thereby form a second overgrown gallium nitride semiconductor layer. Microelectronic devices may then be formed in the second overgrown gallium nitride semiconductor layer.

[0010] It has been found, according to the present invention, that although dislocation defects may propagate vertically from the underlying gallium nitride layer to the grown gallium nitride layer above the first mask openings, the first overgrown gallium nitride layer is relatively defect-free. Moreover, since the second array of mask openings is laterally offset from the first array of mask openings, the relatively defect-free overgrown first gallium nitride layer propagates through the second array of openings and onto the second mask. Accordingly, high performance microelectronic devices may be formed in the second overgrown gallium nitride semiconductor layer.

[0011] According to another aspect of the present invention, the second overgrown gallium nitride semiconductor layer is overgrown until the second overgrown gallium nitride layer coalesces on the second mask, to form a continuous overgrown monocrystalline gallium nitride semiconductor layer. The entire continuous overgrown layer can thus be relatively defect-free compared to the underlying gallium nitride layer.

[0012] The first and second gallium nitride semiconductor layers may be grown using metalorganic vapor phase epitaxy (MOVPE). Preferably, the openings in the masks are stripes that are oriented along the $\langle{1\overset{\_}{1}00}\rangle$

direction of the underlying gallium nitride layer. The overgrown gallium nitride layers may be grown using triethylgallium (TEG) and ammonia (NH₃) precursors at 1000-1100ºC and 45 Torr. Preferably, TEG at 13-39µmol/min and NH₃ at 1500 sccm are used in combination with 3000 sccm H₂ diluent. Most preferably, TEG at 26µmol/min, NH₃ at 1500 sccm and H₂ at 3000 sccm at a temperature of 1100ºC and 45 Torr are used. The underlying gallium nitride layer preferably is formed on a substrate, which itself includes a buffer layer such as aluminum nitride, on a substrate such as 6H-SiC(0001).

[0013] Gallium nitride semiconductor structures according to the present invention include an underlying gallium nitride layer, a first lateral gallium nitride layer that extends from the underlying gallium nitride layer and a second lateral gallium nitride layer that extends from the first lateral gallium nitride layer. A plurality of microelectronic devices are provided in the second lateral gallium nitride layer.

[0014] In a preferred embodiment, gallium nitride semiconductor structures according to the present invention include an underlying gallium nitride layer and a first mask that includes a first array of openings therein, on the underlying gallium nitride layer. A first vertical gallium nitride layer extends from the underlying gallium nitride layer through the first array of openings. A first lateral gallium nitride layer extends from the vertical gallium nitride layer onto the mask, opposite the underlying gallium nitride layer. A second mask on the first lateral gallium nitride layer includes a second array of openings therein that are laterally offset from the first array of openings. A second vertical gallium nitride layer extends from the first lateral gallium nitride layer and through the second array of openings. A second lateral gallium nitride layer extends from the second vertical gallium nitride layer onto the second mask, opposite the first lateral gallium nitride layer. A plurality of microelectronic devices including but not limited to optoelectronic devices and field emitters, are formed in the second vertical gallium nitride layer and in the second lateral gallium nitride layer.

[0015] Preferably, the second lateral gallium nitride layer is a continuous monocrystalline gallium nitride semiconductor layer. The underlying gallium nitride layer includes a predetermined defect density, and the second vertical and lateral gallium nitride semiconductor layers are of lower defect density than the predetermined defect density. Accordingly, continuous low defect density gallium nitride semiconductor layers may be produced, to thereby allow the production of high-performance microelectronic devices, using laterally offset masks.

Brief Description of Drawings

[0016]Figure 1 is a cross-sectional view of gallium nitride semiconductor structures according to the present invention.

[0017] Figures 2-9 are cross-sectional views of structures of Figure 1 during intermediate fabrication steps, according to the present invention.

Detailed Description

[0018] The present invention now will be described more fully hereinafter with reference to the accompanying drawings, in which preferred embodiments of the invention are shown. This invention may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein; rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art. In the drawings, the thickness of layers and regions are exaggerated for clarity. Like numbers refer to like elements throughout. It will be understood that when an element such as a layer, region or substrate is referred to as being "on" another element, it can be directly on the other element or intervening elements may also be present. In contrast, when an element is referred to as being "directly on" another element, there are no intervening elements present. Moreover, each embodiment described and illustrated herein includes its complementary conductivity type embodiment as well.

[0019] Referring now to Figure 1, gallium nitride semiconductor structures according to the present invention are illustrated. The gallium nitride structures 200 include a substrate 102. The substrate may be sapphire or gallium nitride. However, preferably, the substrate includes a 6H-SiC(0001) substrate 102a and an aluminum nitride buffer layer 102b on the silicon carbide substrate 102a. The aluminum nitride buffer layer 102b may 0.01µm thick.

[0020] The fabrication of substrate 102 is well known to those having skill in the art and need not be described further. Fabrication of silicon carbide substrates are described, for example, in U.S. Patents 4,865,685 to Palmour; Re 34,861 to Davis et al.; 4,912,064 to Kong et al. and 4,946,547 to Palmour et al., the disclosures of which are hereby incorporated herein by reference. Also, the crystallographic designation conventions used herein are well known to those having skill in the art, and need not be described further.

[0021] An underlying gallium nitride layer 104 is also included on buffer layer 102b opposite substrate 102a. The underlying gallium nitride layer 104 may be between about 1.0 and 2.0µm thick, and may be formed using heated metalorganic vapor phase epitaxy (MOVPE). The underlying gallium nitride layer generally has an undesired relatively high defect density, for example dislocation densities of between about 10⁸ and 10¹⁰cm⁻². These high defect densities may result from mismatches in lattice parameters between the buffer layer 102b and the underlying gallium nitride layer 104. These high defect densities may impact performance of microelectronic devices formed in the underlying gallium nitride layer 104.

[0022] Still continuing with the description of Figure 1, a first mask such as a first silicon dioxide mask 106 is included on the underlying gallium nitride layer 104. The first mask 106 includes a first array of openings therein. Preferably, the first openings are first stripes that extend along the $\langle{1\overset{\_}{1}00}\rangle$

direction of the underlying gallium nitride layer 104. The first mask 106 may have a thickness of about 1000Åand may be formed on the underlying gallium nitride layer 104 using low pressure chemical vapor deposition (CVD) at 410ºC. The first mask 106 may be patterned using standard photolithography techniques and etched in a buffered hydrofluoric acid (HF) solution.

[0023] Continuing with the description of Figure 1, a first vertical gallium nitride layer 108a extends from the underlying gallium nitride layer 104 and through the first array of openings in the first mask 106. As used herein, the term "vertical" means a direction that is orthogonal to the faces of the substrate 102. The first vertical gallium nitride layer 108a may be formed using metalorganic vapor phase epitaxy at about 1000-1100ºC and 45 Torr. Precursors of triethygallium (TEG) at 13-39µmol/min and ammonia (NH₃) at 1500 sccm may be used in combination with a 3000 sccm H₂ diluent, to form the first vertical gallium nitride layer 108a.

[0024] Still continuing with the description of Figure 1, the gallium nitride semiconductor structure 200 also includes a first lateral gallium nitride layer 108b that extends laterally from the first vertical gallium nitride layer 108a onto the first mask 106 opposite the underlying gallium nitride layer 104. The first lateral gallium nitride layer 108b may be formed using metalorganic vapor phase epitaxy as described above. As used herein, the term "lateral" denotes a direction parallel to the faces of substrate 102.

[0025] As shown in Figure 1, first lateral gallium nitride layer 108b coalesces at first interfaces 108c to form a first continuous monocrystalline gallium nitride semiconductor layer 108. It has been found that the dislocation densities in the first underlying gallium nitride layer 104 generally do not propagate laterally with the same intensity as vertically. Thus, first lateral gallium nitride layer 108b can have a relatively low defect density, for example less that 10⁴cm⁻². It will also be understood that the first lateral gallium nitride layer 108b need not coalesce on the first mask 206.

[0026] Still continuing with the description of Figure 1, a second mask such as a second silicon dioxide mask 206 is included on the first vertical gallium nitride layer 108a. As shown, second mask 206 is laterally offset from first mask 106. It will also be understood that second mask 206 may also extend onto first vertical gallium nitride layer 108b. Preferably, second mask 206 covers all of first vertical gallium nitride layer 108b such that defects in this layer do not propagate further. It will also be understood that the second mask 206 need not be symmetrically offset with respect to first mask 106. The second mask 206 includes a second array of openings therein. The second openings are preferably oriented as described in connection with the first mask 106. The second mask 206 also may be fabricated similar to first mask 106.

[0027] Continuing with the description of Figure 1, a second vertical gallium nitride layer 208a extends from the first lateral gallium nitride layer 108a and through the second array of openings in the second mask 206. The second vertical gallium nitride layer 208a may be formed similar to first vertical gallium nitride layer 108a. The gallium nitride semiconductor structure 200 also includes a second lateral gallium nitride layer 208b that extends laterally from the second vertical gallium nitride layer 208a onto the second mask 206 opposite the first gallium nitride layer 108. The second lateral gallium nitride layer 208b may be formed using metalorganic vapor phase epitaxy as was described above.

[0028] As shown in Figure 1, the second lateral gallium nitride layer 208b coalesces at second interfaces 208c, to form a second continuous monocrystalline gallium nitride semiconductor layer 208. It has been found that since the first lateral gallium nitride layer 108b is used to grow second gallium nitride layer 208, the second gallium nitride layer 208 including second vertical gallium nitride layer 208a and second lateral gallium nitride layer 208b, can have a relatively low defect density, for example less than 10⁴cm⁻². Accordingly, the entire gallium nitride layer 208 can form device quality gallium nitride semiconductor material. Thus, as shown in Figure 1, microelectronic devices 210 may be formed in both the second vertical gallium nitride layer 208a and the second lateral gallium nitride layer 208b, and may bridge these layers as well. By offsetting masks 106 and 206, a continuous device quality gallium nitride layer may be obtained.

[0029] Referring now to Figures 2-9, methods of fabricating gallium nitride semiconductor structures according to the present invention will now be described. As shown in Figure 2, an underlying gallium nitride layer 104 is grown on a substrate 102. The substrate 102 may include a 6H-SiC(0001) substrate 102a and an aluminum nitride buffer layer 102b. The gallium nitride layer 104 may be between 1.0 and 2.0µm thick, and may be grown at 1000ºC on a high temperature (1100ºC) aluminum nitride buffer layer 102b that was deposited on 6H-SiC substrate 102a in a cold wall vertical and inductively heated metalorganic vapor phase epitaxy system using triethylgallium at 26µmol/min, ammonia at 1500 sccm and 3000 sccm hydrogen diluent. Additional details of this growth technique may be found in a publication by T.W. Weeks et al. entitled "GaN Thin Films Deposited Via Organometallic Vapor Phase Epitaxy on α(6H)-SiC(0001) Using High-Temperature Monocrystalline AlN Buffer Layers", Applied Physics Letters, Vol. 67, No. 3, July 17, 1995, pp. 401-403, the disclosure of which is hereby incorporated herein by reference. Other substrates, with or without buffer layers, may be used.

[0030] Still referring to Figure 2, the underlying gallium nitride layer 104 is masked with a first mask 106 that includes a first array of openings 107 therein. The first mask may comprise silicon dioxide at thickness of 1000Å and may be deposited using low pressure chemical vapor deposition at 410ºC. Other masking materials may be used. The first mask may be patterned using standard photolithography techniques and etching in a buffered HF solution. In one embodiment, the first openings 107 are 3µm-wide openings that extend in parallel at distances of between 3 and 40µm and that are oriented along the $\langle{1\overset{\_}{1}00}\rangle$

direction on the underlying gallium nitride layer 104. Prior to further processing, the structure may be dipped in a 50% buffered hydrochloric acid (HCl) solution to remove surface oxides from the underlying gallium nitride layer 104.

[0031] Referring now to Figure 3, the underlying gallium nitride layer 104 is grown through the first array of openings 107 to form first vertical gallium nitride layer 108a in the first openings. Growth of gallium nitride may be obtained at 1000-1100ºC and 45 Torr. The precursors TEG at 13-39µmol/min and NH₃ at 1500 sccm may be used in combination with a 3000 sccm H₂ diluent. If gallium nitride alloys are formed, additional conventional precursors of aluminum or indium, for example, may also be used. As shown in Figure 3, the first gallium nitride layer 108a grows vertically to the top of the first mask 106.

[0032] It will be understood that underlying gallium nitride layer 104 may also be grown laterally without using a mask 106, by appropriately controlling growth parameters and/or by appropriately patterning the underlying gallium nitride layer 104. A patterned layer may be formed on the underlying gallium nitride layer after vertical growth or lateral growth, and need not function as a mask.

[0033] It will also be understood that lateral growth in two dimensions may be used to form an overgrown gallium nitride semiconductor layer. Specifically, mask 106 may be patterned to include an array of openings 107 that extend along two orthogonal directions such as $\langle{1\overset{\_}{1}00}\rangle$

and $\langle{11\overset{\_}{2}0}\rangle$

. Thus, the openings can form a rectangle of orthogonal striped patterns. In this case, the ratio of the edges of the rectangle is preferably proportional to the ratio of the growth rates of the $\left\{ {11\overset{\_}{2}0} \right\}$

and $\left\{ {1\overset{\_}{1}01} \right\}$

facets, for example, in a ratio of 1.4:1.

[0034] Referring now to Figure 4, continued growth of the first gallium nitride layer 108a causes lateral overgrowth onto the first mask 106, to form first lateral gallium nitride layer 108b. Growth conditions for overgrowth may be maintained as was described in connection with Figure 3.

[0035] Referring now to Figure 5, lateral overgrowth is optionally allowed to continue until the lateral growth fronts coalesce at first interfaces 108c, to form a first continuous gallium nitride layer 108. The total growth time may be approximately 60 minutes.

[0036] Referring now to Figure 6, the first vertical gallium nitride layer 108a is masked with a second mask 206 that includes a second array of openings 207 therein. The second mask may be fabricated as was described in connection with the first mask. The second mask may also be eliminated, as was described in connection with the first mask of Figure 3. The second mask may also be eliminated, as was described in connection with Figure 3. As already noted, the second mask 206 preferably covers the entire first vertical gallium nitride layer 108a, so as to prevent defects therein from propagating vertically or laterally. In order to provide defect-free propagation, mask 206 may extend onto first lateral gallium nitride layer 108b as well.

[0037] Referring now to Figure 7, the first lateral gallium nitride layer 108c is grown vertically through the second array of openings 207, to form second vertical gallium nitride layer 208a in the second openings. Growth may be obtained as was described in connection with Figure 3.

[0038] Referring now to Figure 8, continued growth of the second gallium nitride layer 208a causes lateral overgrowth onto the second mask 206, to form second lateral gallium nitride layer 208b. Lateral growth may be obtained as was described in connection with Figure 3.

[0039] Referring now to Figure 9, lateral overgrowth preferably continues until the lateral growth fronts coalesce at second interfaces 208c to form a second continuous gallium nitride layer 208. Total growth time may be approximately 60 minutes. Microelectronic devices may then be formed in regions 208a and in regions 208b as shown in Figure 1, because both of these regions are of relatively low defect density. Devices may bridge these regions as well, as shown. Accordingly, a continuous device quality gallium nitride layer 208 may be formed.

[0040] Additional discussion of the methods and structures of the present invention will now be provided. As described above, the openings 107 and 207 in the masks are preferably rectangular stripes that preferably extend along the $\langle{11\overset{\_}{2}0}\rangle$

and/or $\langle{1\overset{\_}{1}00}\rangle$

directions relative to the underlying gallium nitride layer 104. Truncated triangular stripes having $\left( {1\overset{\_}{1}01} \right)$

slant facets and a narrow (0001) top facet may be obtained for mask openings 107 and 207 along the $\langle{11\overset{\_}{2}0}\rangle$

direction. Rectangular stripes having a (0001) top facet, $\left( {11\overset{\_}{2}0} \right)$

vertical side faces and $\left( {1\overset{\_}{1}01} \right)$

slant facets may be grown along the $\langle{1\overset{\_}{1}00}\rangle$

direction. For growth times up to 3 minutes, similar morphologies may be obtained regardless of orientation. The stripes develop into different shapes if the growth is continued.

[0041] The amount of lateral growth generally exhibits a strong dependence on stripe orientation. The lateral growth rate of the $\langle{1\overset{\_}{1}00}\rangle$

oriented stripes is generally much faster than those along $\langle{11\overset{\_}{2}0}\rangle$

. Accordingly, it is most preferred to orient the openings 107 and 207 so that they extend along the $\langle{1\overset{\_}{1}00}\rangle$

direction of the underlying gallium nitride layer 104.

[0042] The different morphological development as a function of opening orientation appears to be related to the stability of the crystallographic planes in the gallium nitride structure. Stripes oriented along $\langle{11\overset{\_}{2}0}\rangle$

may have wide $\left( {1\overset{\_}{1}00} \right)$

slant facets and either a very narrow or no (0001) top facet depending on the growth conditions. This may be because $\left( {1\overset{\_}{1}01} \right)$

is the most stable plane in the gallium nitride wurtzite crystal structure, and the growth rate of this plane is lower than that of others. The $\left\{ {1\overset{\_}{1}01} \right\}$

planes of the $\langle{1\overset{\_}{1}00}\rangle$

oriented stripe may be wavy, which implies the existence of more than one Miller index. It appears that competitive growth of selected $\left\{ {1\overset{\_}{1}01} \right\}$

planes occurs during the deposition which causes these planes to become unstable and which causes their growth rate to increase relative to that of the $\left( {1\overset{\_}{1}01} \right)$

of stripes oriented along $\langle{11\overset{\_}{2}0}\rangle$

.

[0043] The morphologies of the gallium nitride layers selectively grown on openings oriented along $\langle{1\overset{\_}{1}00}\rangle$

are also generally a strong function of the growth temperatures. Layers grown at 1000ºC may possess a truncated triangular shape. This morphology may gradually change to a rectangular cross-section as the growth temperature is increased. This shape change may occur as a result of the increase in the diffusion coefficient and therefore the flux of the gallium species along the (0001) top plane onto the $\left\{ {1\overset{\_}{1}01} \right\}$

planes with an increase in growth temperature. This may result in a decrease in the growth rate of the (0001) plane and an increase in that of the $\left\{ {1\overset{\_}{1}01} \right\}$

. This phenomenon has also been observed in the selective growth of gallium arsenide on silicon dioxide. Accordingly, temperatures of 1100ºC appear to be most preferred.

[0044] The morphological development of the gallium nitride regions also appears to depend on the flow rate of the TEG. An increase in the supply of TEG generally increases the growth rate of the stripes in both the lateral and the vertical directions. However, the lateral/vertical growth rate ratio decrease from 1.7 at the TEG flow rate of 13µmol/min to 0.86 at 39µmol.min. This increased influence on growth rate along <0001> relative to that of $\langle{11\overset{\_}{2}0}\rangle$

with TEG flow rate may be related to the type of reactor employed, wherein the reactant gases flow vertically and perpendicular to the substrate. The considerable increase in the concentration of the gallium species on the surface may sufficiently impede their diffusion to the $\left\{ {1\overset{\_}{1}01} \right\}$

planes such that chemisorption and gallium nitride growth occur more readily on the (0001) plane.

[0045] Continuous 2µm thick gallium nitride layers 108 and 208 may be obtained using 3µm wide stripe openings 107 and 207 spaced 7µm apart and oriented along $\langle{1\overset{\_}{1}00}\rangle$

, at 1100ºC and a TEG flow rate of 26µmol/min. The overgrown gallium nitride layers 108b and 208b may include subsurface voids that form when two growth fronts coalesce. These voids may occur most often using lateral growth conditions wherein rectangular stripes having vertical $\left\{ {11\overset{\_}{2}0} \right\}$

side facets developed.

[0046] The coalesced gallium nitride layers 108 and 208 may have a microscopically flat and pit-free surface. The surfaces of the laterally grown gallium nitride layers may include a terrace structure having an average step height of 0.32nm. This terrace structure may be related to the laterally grown gallium nitride, because it is generally not included in much larger area films grown only on aluminum nitride buffer layers. The average RMS roughness values may be similar to the values obtained for the underlying gallium nitride layers 104.

[0047] Threading dislocations, originating from the interface between the gallium nitride underlayer 104 and the buffer layer 102b, appear to propagate to the top surface of the first vertical gallium nitride layer 108a within the first openings 107 of the first mask 106. The dislocation density within these regions is approximately 10⁹cm⁻². By contrast, threading dislocations do not appear to readily propagate into the first overgrown regions 108b. Rather, the first overgrown gallium nitride regions 108b contain only a few dislocations. These few dislocations may be formed parallel to the (0001) plane via the extension of the vertical threading dislocations after a 90º bend in the regrown region. These dislocations do not appear to propagate to the top surface of the first overgrown GaN layer. Since both the second vertical gallium nitride layer 208a and the second lateral gallium nitride layer 208b propagate from the low defect first overgrown gallium nitride layer 108b, the entire layer 208 can have low defect density.

[0048] As described, the formation mechanism of the selectively grown gallium nitride layer is lateral epitaxy. The two main stages of this mechanism are vertical growth and lateral growth. During vertical growth, the deposited gallium nitride grows selectively within the mask openings 107 and 207 more rapidly than it grows on the masks 106 and 206, apparently due to the much higher sticking coefficient, s, of the gallium atoms on the gallium nitride surface (s=1) compared to on the mask (s~1). Since the SiO₂ bond strength is 799.6 kJ/mole and much higher than that of Si-N (439 kJ/mole), Ga-N (103 kJ/mole), and Ga-O (353.6 kJ/mole), Ga or N atoms should not readily bond to the mask surface in numbers and for a time sufficient to cause gallium nitride nuclei to form. They would either evaporate or diffuse along the mask surface to the openings 107 or 207 in the masks or to the vertical gallium nitride surfaces 108a or 208a which have emerged. During lateral growth, the gallium nitride grows simultaneously both vertically and laterally over the mask from the material which emerges over the openings.

[0049] Surface diffusion of gallium and nitrogen on the masks may play a minor role in gallium nitride selective growth. The major source of material appears to be derived from the gas phase. This may be demonstrated by the fact that an increase in the TEG flow rate causes the growth rate of the (0001) top facets to develop faster than the $\left( {1\overset{\_}{1}01} \right)$

side facets and thus controls the lateral growth.

[0050] The laterally grown gallium nitride layers 108b and 208b bond to the underlying masks 106 and 206 sufficiently strongly so that they generally do not break away on cooling. However, lateral cracking within the SiO₂ may take place due to thermal stresses generated on cooling. The viscosity ρ of the SiO₂ at 1050ºC is about 10^(15.5) poise which is one order of magnitude greater than the strain point (about 10^(14.5) poise) where stress relief in a bulk amorphous material occurs within approximately six hours. Thus, the SiO₂ mask may provide limited compliance on cooling. As the atomic arrangement on the amorphous SiO₂ surface is quite different from that on the GaN surface, chemical bonding may occur only when appropriate pairs of atoms are in close proximity. Extremely small relaxations of the silicon and oxygen and gallium and nitrogen atoms on the respective surfaces and/or within the bulk of the SiO₂ may accommodate the gallium nitride and cause it to bond to the oxide.

[0051] Accordingly, regions of lateral epitaxial overgrowth through mask openings from an underlying gallium nitride layer may be achieved via MOVPE. The growth may depend strongly on the opening orientation, growth temperature and TEG flow rate. Coalescence of overgrown gallium nitride regions to form regions with both extremely low densities of dislocations and smooth and pit-free surfaces may be achieved through 3µm wide mask openings spaced 7µm apart and extending along the $\langle{1\overset{\_}{1}00}\rangle$

direction, at 1100ºC and a TEG flow rate of 26µmol/min. The lateral overgrowth of gallium nitride via MOVPE may be used to obtain low defect density continuous gallium nitride layers for microelectronic devices.

[0052] In the drawings and specification, there have been disclosed typical preferred embodiments of the invention and, although specific terms are employed, they are used in a generic and descriptive sense only and not for purposes of limitation, the scope of the invention being set forth in the following claims. 

Claims
 1. A method of fabricating a gallium nitride semiconductor layer comprising the steps of: masking an underlying gallium nitride layer with a first mask that includes a first array of openings therein; growing the underlying gallium nitride layer through the first array of openings and onto the mask to thereby form a first overgrown gallium nitride semiconductor layer; masking the first overgrown gallium nitride layer with a second mask that includes a second array of openings therein, the second array of openings being laterally offset from the first array of openings; and growing the first overgrown gallium nitride layer through the second array of openings and onto the second mask, to thereby form a second overgrown gallium nitride semiconductor layer.
 2. A method according to Claim 1 wherein the step of growing the first overgrown gallium nitride layer is followed by the step of forming at least one microelectronic device in the second overgrown gallium nitride semiconductor layer.
 3. A method according to Claim 1 wherein the step of growing the first overgrown gallium nitride layer comprises the step of growing the first overgrown gallium nitride layer through the second array of openings and onto the second mask until the second overgrown gallium nitride layer coalesces on the second mask to form a continuous overgrown monocrystalline gallium nitride semiconductor layer.
 4. A method according to Claim 1 wherein the growing steps comprise the steps of growing the underlying gallium nitride layer and growing the first overgrown gallium nitride layer using metalorganic vapor phase epitaxy.
 5. A method according to Claim 1 wherein the first masking step is preceded by the step of forming the underlying gallium nitride layer on a substrate.
 6. A method according to Claim 5 wherein the forming step comprises the steps of: forming a buffer layer on a substrate; and forming the underlying gallium nitride layer on the buffer layer opposite the substrate.
 7. A method according to Claim 1 wherein the first and second masking steps comprise the steps of:masking the underlying gallium nitride layer and the first overgrown gallium nitride layer with a first mask and a second mask respectively, that include respective first and second arrays of stripe openings therein, the stripe openings extending along a $\langle{1\overset{\_}{1}00}\rangle$

direction of the underlying gallium nitride layer.
 8. A method according to Claim 1 wherein the underlying gallium nitride layer includes a predetermined defect density, and wherein the step of growing the underlying gallium nitride layer through the first array of openings and onto the mask to thereby form a first overgrown gallium nitride semiconductor layer comprises the steps of: vertically growing the underlying gallium nitride layer through the first array of openings while propagating the predetermined defect density; and laterally growing the underlying gallium nitride layer from the first array of openings onto the first mask to thereby form a first overgrown gallium nitride semiconductor layer of lower defect density than the predetermined defect density.
 9. A method according to Claim 8 wherein the step of growing the first overgrown gallium nitride layer comprises the steps of: vertically growing the first overgrown gallium nitride semiconductor layer through the second array of openings; and laterally growing the first overgrown gallium nitride semiconductor layer from the second array of openings onto the second mask, to thereby form a second overgrown gallium nitride semiconductor layer of lower defect density than the predetermined defect density.
 10. A method according to Claim 1 wherein the underlying gallium nitride layer includes a predetermined defect density, and wherein the second overgrown gallium nitride semiconductor layer is of lower defect density than the predetermined defect density.
 11. A method according to Claim 1 wherein the growing steps comprise the steps of growing the underlying gallium nitride layer and growing the first overgrown gallium nitride layer using metalorganic vapor phase epitaxy of triethylgallium at 13-39µmol/min and ammonia at 1500 sccm at a temperature of 1000ºC-1100ºC.
 12. A method according to Claim 7 wherein the steps of growing the underlying gallium nitride layer and growing the first overgrown gallium nitride layer comprise the steps of growing the underlying gallium nitride layer and the first overgrown gallium nitride layer using metalorganic vapor phase epitaxy of triethylgallium at 26µmol/min and ammonia at 1500 sccm at a temperature of 1100ºC.
 13. A method of fabricating a gallium nitride semiconductor layer comprising the steps of: laterally growing an underlying gallium nitride layer to thereby form a first laterally grown gallium nitride semiconductor layer; and laterally growing the first laterally grown gallium nitride layer, to thereby form a second laterally grown gallium nitride semiconductor layer.
 14. A method according to Claim 13 wherein the step of laterally growing the first laterally grown galliumm nitride layer is followed by the step of forming at least one microelectronic device in the second laterally grown gallium nitride semiconductor layer.
 15. A method according to Claim 13 wherein the step of laterally growing the first laterally grown gallium nitride layer comprises the step of laterally growing the first laterally grown gallium nitride layer until the second laterally grown gallium nitride layer coalesces to form a continuous laterally grown monocrystalline gallium nitride semiconductor layer.
 16. A method according to Claim 13 wherein the laterally growing steps comprise the steps of laterally growing the underlying gallium nitride layer and laterally growing the first laterally grown gallium nitride layer using metalorganic vapor phase epitaxy.
 17. A method according to Claim 13 wherein the step of laterally growing the first laterally grown gallium nitrde layer comprises the step of laterally overgrowing the first laterally grown gallium nitride layer.
 18. A method according to Claim 13 wherein the underlying gallium nitride layer includes a predetermined defect density, and wherein the step of laterally growing the first laterally grown gallium nitride layer comprises the step of: laterally growing the first laterally grown gallium nitride semiconductor layer, to thereby form a second laterally grown gallium nitride semiconductor layer of lower defect density than the predetermined defect density.
 19. A method according to Claim 2 wherein the step of forming at least one microelectronic device in the second overgrown gallium nitride semiconductor layer comprises the step of forming at least one microelectronic device in the second overgrown gallium nitride semiconductor layer and in the first overgrown gallium nitride layer.
 20. A method according to Claim 14 wherein the step of forming at least one microelectronic device in the second laterally grown gallium nitride semiconductor layer comprises the step of forming at least one microelectronic device in the second laterally grown gallium nitride semiconductor layer and in the first laterally grown gallium nitride semiconductor layer.
 21. A method according to Claim 1 wherein the first and second arrays of openings extend along a $\langle{11\overset{\_}{2}0}\rangle$

direction of the underlying gallium nitride layer.
 22. A method according to Claim 1 wherein the first and second arrays of openings extend along a $\langle{11\overset{\_}{2}0}\rangle$

direction of the underlying gallium nitride layer, the first mask and the second mask also including respective third and fourth arrays of openings therein that extend along a $\langle{1\overset{\_}{1}00}\rangle$

direction of the underlying gallium nitride layer.
 23. A method according to Claim 22 wherein the first and third arrays of openings and the second and fourth arrays of openings are arranged in rectangles on the underlying gallium nitride layer and on the first overgrown gallium nitride semiconductor layer, respectively, the openings having edges of predetermined lengths, and wherein a ratio of the predetermined lengths is proportional to a ratio of growth rates of a $\left\{ {11\overset{\_}{2}0} \right\}$

facet and a $\left\{ {1\overset{\_}{1}01} \right\}$

facet of the underlying gallium nitride layer.
 24. A method according to Claim 13 wherein the first laterally grown gallium nitride semiconductor layer includes spaced apart stripes that extend along a $\langle{1\overset{\_}{1}00}\rangle$

direction of the underlying gallium nitride layer.
 25. A method according to Claim 13 wherein the first laterally grown gallium nitride semiconductor layer includes spaced apart stripes that extend along a $\langle{11\overset{\_}{2}0}\rangle$

direction of the underlying gallium nitride layer.
 26. A method according to Claim 13 wherein the first laterally grown gallium nitride semiconductor layer includes an array of first spaced apart regions that extend along a $\langle{11\overset{\_}{2}0}\rangle$

direction of the underlying gallium nitride layer, and an array of second spaced apart regions that extend along a $\langle{1\overset{\_}{1}00}\rangle$

direction of the underlying gallium nitride layer.
 27. A method according to Claim 26 wherein the array of first spaced apart regions and the array of second spaced apart regions are arranged in a rectangle in the first laterally grown gallium nitride semiconductor layer, the rectangle having edges of predetermined lengths, and wherein a ratio of the of the predetermined lengths is proportional to a ratio of growth rates of a $\left\{ {11\overset{\_}{2}0} \right\}$

facet and a $\left\{ {1\overset{\_}{1}01} \right\}$

facet of the first laterally grown gallium nitride semiconductor layer. 